Boundary Layer Clouds

The project S2 aims at exploiting the unique research opportunities created during Phase I of HD(CP)² to improve the parameterization of boundary-layer physics, radiation and low level cloud microphysics in climate models and to investigate the response of clouds to climate change. The aim is to gain more insight into the inner workings of the system of interacting fast physics, and its response when subject to an idealized external forcing of CO2 quadrupling.

In work package 1, first steps towards the comparison between model output and observations have been done. Observed liquid cloud fraction profiles over the supersites of JOYCE and RAO Lindenberg have been derived and compared to the liquid cloud fraction profiles obtained from each of the different domains of the ICON-LEM model output.

The aim of work package 2 is to assess the capability of the ICON-LEM to realistically simulate low-level clouds and to accurately represent the resulting cloud radiative effects. Towards this direction, four diagnostic quantities have to be considered: optical thickness, statistical distribution of the liquid water content within the grid, geometrical cloud thickness, and cloud base and top altitudes.

Within work package 3, the evaporation of precipitation will be analysed in detail, using temperature and humidity profiles from supersites. The availability of passive and active remote sensing data for a longer time period will be used to find systematic features which should lead in improved parameterizations for evaporation in models.

The overall aim of work package 4 is to improve cloud representation and cloud radiative interactions in the ICON Global Climate Model by implementing and developing a so called probability density function (PDF) schemes. The core idea of PDF schemes (also sometimes referred to as statistical schemes) is to represent the sub-grid scale variability of one or multiple variables through an analytical PDF for each model cell.

Within work package 5 we aim at systematically characterizing the effect of radiation on boundary layer cloud development on the ICON-LEM resolution, and at studying the relevance for the fast cloud feedback using ICON-GCM. In Phase I of HD(CP)², accurate 3D radiativ etransfer parameterizations of these effects have been developed (Jakub and Mayer, 2015a; Klinger and Mayer, 2016) which are fast enough to be included into LEM runs, at least for limited time periods.

Work package 6.1 employs the ICON model system at a range of resolutions and domain sizes, from local cloud-resolving (ICON-LEM) to global "climate-permitting" (ICON-GCM).

The main aim of work package 6.2 is to determine the effects elevated moisture layers have on low level boundary layer clouds. Present day cases of observed elevated moisture layers are being used as proxy cases to represent future atmospheric conditions resulting from climate change.

Work package 7 focuses on data-driven investigations of interactions between turbulence, small-scale non-turbulent motions and boundary-layer clouds. At nighttime, boundary-layer turbulence is highly intermittent and poorly parameterized. The intermittency of turbulence is related partly to local forcing by small-scale wind accelerations that are typically not resolved in numerical models, but also on larger time scales to cloud cover and geostrophic wind.